Dicliny or unisexuality of the flower ensures that only cross-pollination. In many bisexual flowers of the plants, stamens and carpels of a flower do not mature at the same time. This is called dichogamy. In some plants, morphological barriers are formed in between the anther and stigma of the same flower, so self-pollination cannot occur. This is called herkogamy. In cleistogamy, flowers do not open, thus ensuring only self-pollination.